https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104931
--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #3)
> The bug was made latent by g:51d464b608b38b9e2007948d10b1e0f1dcec142c which
> ensured that
>
> /* If the loop exits immediately, there is nothing to do. */
> tree tem = fold_binary (code, boolean_type_node, iv0->base, iv1->base);
> if (tem && integer_zerop (tem))
> {
> if (!every_iteration)
> return false;
> niter->niter = build_int_cst (unsigned_type_for (type), 0);
> niter->max = 0;
> return true;
> }
>
> triggered, folding (_2 + 4294967272) + 12 < _2 + 4294967272 to false. That's
> the following part of the revision, and it probably triggers when doing
> expand_simple_operations.
>
> diff --git a/gcc/match.pd b/gcc/match.pd
> index 84c9b918041..f5dcbf32bc7 100644
> --- a/gcc/match.pd
> +++ b/gcc/match.pd
> @@ -2143,6 +2143,11 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
> (simplify
> (pointer_plus (pointer_plus:s @0 @1) @3)
> (pointer_plus @0 (plus @1 @3)))
> +#if GENERIC
> +(simplify
> + (pointer_plus (convert:s (pointer_plus:s @0 @1)) @3)
> + (convert:type (pointer_plus @0 (plus @1 @3))))
> +#endif
>
> /* Pattern match
>
> It does seem to me that niter analysis relies on statically detecting
> not rolling loops, at least for the cases we assume no overflow happens
> and we use number_of_iterations_lt_to_ne. I can't convince myself that
> only INTEGER_CST appearant negative delta are problematic for
> pointer types (which we always assume to have no overflow), but that would
> be the most simplistic solution here. Still "negative" delta values should
> be problematic for all cases, and since we only restrict us to constant
> modulo, delta can also be non-constant.
I verified backporting the above fixes the issue. That's really the patch
I'm most comfortable with at this point ... :/
